rpm vs. speed question

There's no precise relationship. It's simply the choice of gearing, presumably matched to the car's intended use, but also affected by marketing and fashion. The Miata doesn't start to make serious power until it reaches 4000 rpm; it also will run all day above 6000 without problems. It's how a small engine delivers good performance, which the factory assumes the buyer will value over quiet cruising. The Miata doesn't have much oomph down low, and more transmission gears makes it easier to stay within the narrower powerband.

OTOH, modern Corvettes have 6 speeds, just because it's fashionable. They could probably get by with two.

I agree with everything Lanny said. But the simplest relationship might be: smaller engine, higher rpm's.

Engine torque is pretty much proportional to displacement (not considering force-induction engines which effectively multiplies displacement). So to make the same power which equals torque*rpm, they have to spin faster.

If all else is the same with a car (weight, size, etc, which obviously it isn't here), it will take the same power to drive any given combination of speed and road grade. Thus the smaller engine must turn higher rpm.

Now, in your case, it is force-induction, and makes a fair amount of torque. And its in a small car. So it should be able to be geared longer and turn lower rpm than 3000 @ 60mph. Lanny is correct that Mazda however chose to keep the rpms high in consideration of the sporting nature of the car. Also, it helps mask turbo lag. Basically, it makes the car more responsive, but at sacrifice of noise, fuel economy, and potentially some engine life. Frankly, especially given it is a 6-speed, I would have much preferred if Mazda made 6th much taller.

Another reason why a manual might be geared shorter than an automatic is for perceived drivability. Imagine if it turned

2000rpm at 60. You press on the pedal ... nothing. You have to downshift. And automatic would do that for you, uh, automatically. Thus perceived better drivability.

And, as Lanny also said, 6-speeds are in vogue. A Corvette, or any similar torque-monster, simply doesn't need them. Lexus is coming out with an 8-speed automatic. Why? Because MB has seven!

Oy.

Anyway, enjoy the car. If the noise gets to be a hassle on a highway cruise, do what a lot of us do: earplugs.

The key de terming factor in the gearing of an engine is the torque the engine produces.

Push rod V 8 engines are designed to produce high torque, partly because the push rod design limits the design to 2 valve per cylinder. This design, while economical, limits the rpm that the engine can turn. They usually red line at relatively low rpm (5000 to 6000 rpm)

Modern fours, using overhead cams can uses three or four lighter valves with better modern spring, can turn 7000 or 8000 rmp. But since the are smaller displacement they produce much lower torque.

The formula for horsepower is:

Horsepower = torque * revs/minute * minute/60 seconds * 2*pi * 1/550 Horsepower = torque * revs/minute * 1/5252

In a very simple way:

Suppose your car requires 100 hp to move along at 90 mph.

A v-8 rated at 300 ft-lb at 2000 rpm will be producing 114. An engineer will pick the gearing and rear end to produce the right rpm at the wheels.

A smaller 4 cylinder engine, producing 140 ft-lbs or torque would have to turn nearly 4300 rpm to produce the power necessary to move the same car at the same 90 mph.

As for the gears, the turbo engine puts out 28 % more power than the normal engine, but the full turbo pressure does not kick in until 4500 rpm. the red line is at 6500, so the maximum power band fairly narrow. If your gears are too wide, you can fall off the power band when you up shift, so having more gears allow you to keep the engine turning at high rpm while you accelerate. (its also more fun)

All good points but I agree with Dave, 6th should be a taller / more relaxed cruising gear. I think the car has plenty of power / torque to pull a somewhat taller top gear and it would be a big improvement in highspeed cruising comfort and in real world highway gas mileage. Both of which the miata realistically falls down a bit on.

Chris

99BBB

Agreed, 6th is a cruising gear. It shouldn't need high revs. If you need to be accelerating hard in overdrive, you need to rethink your driving habits and learn to downshift. That was one of my few major complaints about my '01.

Eric Lucas

6th is not a cruising gear in the Mazdaspeed Miata.

The car redlines in 5th at 107 and 6500 rpm, and 6th, while an overdrive, gets you to 127. The torque on the vehicle is ok on the lower end, but the fun does not start until 4000 rpm (See

under 2006 MX-5 versus 2004Mazdaspeed for the graph) To get more torque at the low end, you need more displacement, and a differently geared transmission. For example, the Pontiac Solstice has a 2.3 liter engine that is rated at 26 more ft-lbs than the NC mill. That car hits its published 120 mph top speed at 6200 rpm in

4th gear. 5th would reach that same 120 t>The trouble is -- the market! Flat country = lower RPM needed. (Australia)

In none of the Miatas. Which is just a waste since the highest gears are useless anyway from a performance standpoint in a Miata.

Leon, occasionally succeeding not to back out in sixth.

That depends on your concept of performance. When I was trying to keep up with the turbo crowd at the final Western Colorado Strafing Run, fifth came in pretty handy when I ran out of rpm in fourth. At 6000 feet, a taller fifth would not have extended my speed past the limiter in fourth, nor pulled the slightest uphill grade. Same thing at track days.

I'd like to see some data that a taller top gear actually saves fuel, given the Miata's utter lack of aerodynamic efficiency. I suspect the only advantage would be a small reduction in noise. And not even that with the top down.

What are you saying, that you never cruise? You're always accelerating? You cruise in some other gear than 6th?

My point is, if you're running it up thru the gears (in the real world), you've usually finished impressing yourself or your friend at the 107mph point but, you know........... is that redlined in 6th or hitting the (top speed) wall prior to redline? Maybe they should scoot 5th and 6th up a bit and increase top speed to 135 or so, hmmmmm.

Chris

99BBB

If I remember correctly when doing my research on the 2006 models, the AT, 5 speed, and 6 speed had final drive ratios of .58, .81, .83 respectively. This would leave the 6 speed running at higher rpms at cruising speed than the other two. In a small engine, if one does a lot of cruising, lower rpms are generally better for both engine life an fuel economy. When I bought my

2001 recently, I really wanted the AT, but couldn't locate one in decent shape because so few of them are available up here. I ended up with the 6 speed because I wanted a LS, and so far have regretted it. Coupled with the clutch shudder that the previous owner never fixed under warranty, and the high cruising RPM which really surprized me, I thing I would have been better off with a 5 speed or the AT. I feel the 6 speed should have been marketed as a "Performance Option" coupled with the performance suspension, rather than the only manual transmission in the more expensive models. I just find the 6 spd lessens my driving experience overall. If I could get a free swap for either of the other transmissions, I would jump at the opportunity.

Boreal

2001 LS Silver/tan

Probably hard to come up with the data unless someone is willing to do a swap. But the physics backs up the expectation. The aero drag stays the same, thus the same load on the wheels. But at lower rpm, there will be less drivetrain losses, and less engine friction, thus less power needed at the pistons. Plus at lower rpm at near the same power, the throttle would be more wide open meaning less pumping losses, and thus a more efficient engine. All would suggest better fuel economy with taller gearing.

At 60, 70, or 80 mph, the Miata is still far from wide open throttle. Lower rpm and more throttle would almost certainly lead to better mpg.

Just FYI, the overall ratio of the 01's were such that the 6-speed does rev a bit lower in top gear than the 5-speed (due to the FD of

3.909 vs 4.3). But I hear ya on the rest.

That's assuming a flat torque curve; you know better than that. If the taller top gear drops the tach off the cam, you'll be using a LOT more throttle, at a lot lower efficiency. You'll be downshifting for hills, too.

Well, only the 10AE has 3.909 : 1 in the rear. The NB8B 6 speeds have

3.636 : 1 in the rear. My 2003 LE 6 speed is 400 RPM lower at 60 MPH than my '91 1.6 L was. Please remember, RPM "IS" fuel comsumption !!! I have spent over 5,000 hours behind large piston engines, and my job was fuel management ! The lower the RPM's I could run and keep the air speed that I wanted, the longer I could stay in the air. (yes, the aero-drag was the same for any power setting !) Our Miata FI system will only let the proper amount of fuel into the engine even at full open. (*)

If you want gas milage, go drive in a flat freeway at 45 ot 55 MPH and don't change the power setting. I am guessing not too many of you want to drive that way,

(*) If you could control the RPM "and" the Manifold Pressure (boost), you could do even better.

Bruce Bing '03 LE (10,000 + hours of piston engine flying time)

Thanks for the vote of confidence :-)

First, let's make it clear we are talking about small FE differences. I think you'd see a 28 mpg car become a 29 or maybe 30 mpg.

Now, most brake specific fuel consumption maps for SI engines have their minimum (most efficient) point at very high throttle - about

70-80%. And at lower than peak rpm. And it is significantly less sensitive to rpm than throttle. I'm staring at a bunch of maps as I type this. So a LOT more throttle is a GOOD thing, as far as fuel efficiency is concerned.

To be sure, the Miata engine could be so peaky that you go too low in the rpm and slip back into a low efficiency region, but I think this is highly, highly unlikely. Most curves I've seen, even for small high-revvers, don't have eff drop before

My '01brochure lists 3.909 for all 6-speed FD's.

It doesn't do throttle enrichment at WOT?

There's an experiment to prove it in! Go steady 40 mph in 4th. Repeat in 5th. Then 6th. Repeat at 50 mph. Repeat at 60 mph. Repeat at 70 mph. Plot results. Anyone game? Anyone? ...

I recently bought a ScanGauge <

> that actually allows instantaneous RPM/MPG readings, among many other things. I currently have it hooked up to my new Ridgeline because of poor fuel economy issues I am having with it. But when/if I get a chance, I may be able to do some testing on my 6 spd and come up with the plots you suggest. I'll post to the group if I ever get around to doing it. Boreal

2001 LS 6 speed Silver/tan

So few (US) Miata owners run out of rpm in 4th except in rare circumstances. And keeping up with turbos when you are running out of rpm in 4th seems like an exercise in futility even in those rare circumstances. You are spending most of your power on drag, having little left for acceleration; they have plenty left.

I do not have the data, but the claim that it has anything to do with aerodynamic efficiency is ridiculous. It has to do with pumping losses. The only way for an engine to be efficient when it is operating at less than its maximum work per stroke is to shut down a few cylinders. Dragging air past a valve to reduce its density by a significant percentage is a major loss of power.

I suspect you have no clue.

Leon